1. Field of the Invention
This invention relates generally to burn in of I/C chips or similar devices, and more particularly to I/C chip burn in utilizing voltage control responsive to some measured value relating to the power and/or temperature of the chip being burned in.
2. Background Information
According to conventional prior art practice, electronic devices, such as I/C chips, often need to be burned in before use at increased voltages and/or temperatures which accelerates early life failures, thus increasing product reliability. Conventional techniques for temperature control during burn in depend on the power generated by the I/C device during burn in. For low powered devices, the temperature of the fluid flowing around the parts is controlled. For medium power devices, each device is contacted by a heat sink. For high power devices, fluid flow, and/or heaters within each heat sink, are controlled by a microprocessor to maintain each device at the desired burn in temperature (typically 120° C. or 140° C.). According to conventional practice, a plurality of devices are burned in at the same time by applying the same voltage and stimulation patterns to each device, and controlling the temperature as indicated above. An example of a commercially available burn in tool that operates by controlling the temperature of the heat sink for each device is the MCC model HPB 2 manufactured by Micro Control Company of Minneapolis, Minn., USA. The major advantage to the prior art is in massively parallel burn in. There are hundreds or even thousands of devices burned in at the same time using the same temperatures, voltages and patterns. Parallel burn in is desired because each part may need to be burned in for tens of hours.
There are some major limitations to burn in of high power devices. Generally, the tool will have a limited amount of current that can be applied to each device. For example, there may be an individual 50 amp power supply for each device in the tool. Another limitation is a limited ability of the tool to remove the resulting heat from each part. For example, the tool may use liquid cooled heat sinks. If the liquid is 20° C., the burn in temperature is 140° C. and the thermal resistance from the liquid through the heat sink to the device is 1.2 C/W, then the maximum power that can be removed is (140−20)/1.2=100 Watts.
There are some major challenges and drawbacks to direct temperature control during high power burn in. One drawback is that higher voltages and temperature levels during burn in can result in power dissipation levels that are higher than would be seen by the devices at normal operating temperatures and power densities which will overload the capacity of the tool. Another problem is that there may be large power variations from device to device, even in devices from the same wafer, e.g. at the same voltage, one device may operate at two to three times higher power than a second device, which drives the need to individually control the temperature of each heat sink. In a related problem, with the prior art technique, it is common for some of the devices being burned in to exceed the current or power limits of the tool, which results in the power to each such device being interrupted by either a fuse or circuit breaker, or by just turning off the power with the appropriate software. These devices do not receive the required burn in, resulting in reduced yield and tool productivity. One conventional solution to this problem is to reduce the voltage that is applied to all parts in the tool until substantially all the parts in the tool are below the current and power limits. The problem is that when the voltage level is reduced, the devices must be burned in for an even longer time to obtain the same overall population reliability. With conventional tooling, it is sometimes necessary to sort devices into groups according to their power or current and burn in each group at a different voltage and duration. Regardless, in conventional tooling, there is invariably a large percentage of devices in each tool that are operating at currents and powers significantly below the tool limits. Thus, burn in duration must be extended. Therefore, it is desired to control burn in to overcome these problems.